Backport changes to topic subscription

Allow either to use assign or subscribe

With `assign` it is not possible to use the rebalancing from Kafka. So if no offset is set we use `subscribe` and rebalancing is possible and otherwise if a offset is set we are using assign.

@see commit d2fa178
@see #570

Author: Steveb-p

pkg/rdkafka/RdKafkaConsumer.php

@@ -99,11 +99,15 @@ public function getQueue()
     public function receive($timeout = 0)
     {
         if (false == $this->subscribed) {
-            $this->consumer->assign([new TopicPartition(
-                $this->getQueue()->getQueueName(),
-                $this->getQueue()->getPartition(),
-                $this->offset
-            )]);
+            if (null === $this->offset) {
+                $this->consumer->subscribe([$this->getQueue()->getQueueName()]);
+            } else {
+                $this->consumer->assign([new TopicPartition(
+                    $this->getQueue()->getQueueName(),
+                    $this->getQueue()->getPartition(),
+                    $this->offset
+                )]);
+            }
 
             $this->subscribed = true;
         }